The Dynamic Effects of Isosteviol on Insulin Secretion and Its Inability to Counteract the Impaired β-Cell Function during Gluco-, Lipo-, and Aminoacidotoxicity: Studies In Vitro

Abstract

Isosteviol (ISV), a diterpene molecule, is an isomer of the backbone structure of a group of substances with proven antidiabetic capabilities. The aim of this study was to investigate if ISV elicits dynamic insulin release from pancreatic islets and concomitantly is able to ameliorate gluco-, lipo-, and aminoacidotoxicity in clonal β-cell line (INS-1E) in relation to cell viability and insulin secretion. Isolated mice islets placed into perifusion chambers were perifused with 3.3 mM and 16.7 mM glucose with/without 10−7 M ISV. INS-1E cells were incubated for 72 h with either 30 mM glucose, 1 mM palmitate or 10 mM leucine with or without 10−7 M ISV. Cell viability was evaluated with a Cytotoxic Fluoro-test and insulin secretion was measured in Krebs-Ringer Buffer at 3.3 mM and 16.7 mM glucose. In the presence of 3.3 mM glucose, 10−7 M ISV did not change basal insulin secretion from perifused islets. However, at a high glucose level of 16.7 mM, 10−7 M ISV elicited a 2.5-fold increase (−ISV: 109.92 ± 18.64 ng/mL vs. +ISV: 280.15 ± 34.97 ng/mL; p < 0.01). After 72 h gluco-, lipo-, or aminoacidotoxicity in INS-1E cells, ISV treatment did not significantly affect cell viability (glucotoxicity, −ISV: 19.23 ± 0.83%, +ISV: 18.41 ± 0.90%; lipotoxicity, −ISV: 70.46 ± 3.15%, +ISV: 65.38 ± 2.81%; aminoacidotoxicity: −ISV: 8.12 ± 0.63%; +ISV: 7.75 ± 0.38%, all nonsignificant). ISV did not improve impaired insulin secretion (glucotoxicity, −ISV: 52.22 ± 2.90 ng/mL, +ISV: 47.24 ± 3.61 ng/mL; lipotoxicity, −ISV: 19.94 ± 4.10 ng/mL, +ISV: 22.12 ± 3.94 ng/mL; aminoacidotoxicity: −ISV: 32.13 ± 1.00 ng/mL; +ISV: 30.61 ± 1.54 ng/mL, all nonsignificant). In conclusion, ISV acutely stimulates insulin secretion at high but not at low glucose concentrations. However, ISV did not counteract cell viability or cell dysfunction during gluco-, lipo-, or aminoacidotoxicity in INS-1E cells.

Steviol glucuronide, a metabolite of steviol glycosides, potently stimulates insulin secretion from isolated mouse islets: Studies in vitro

“…Our group previously demonstrated that stevioside reduces postprandial blood glucose levels in type 2 diabetic subjects, 4 and suppresses glucose response to a glucose tolerance test, increases insulin secretion, reduces glucagon release and lowers blood pressure in type 2 diabetic Goto-Kakizaki rats. 8,9,13,23 In the present study, we have found that the metabolite, SVG, possesses the same properties. When supplemented to soy-based diet, stevioside shows beneficial effects on the development of the metabolic syndrome with hyperglycaemia, hypertension and dyslipidaemia.…”

“…10,28 Furthermore, we have disclosed that stevioside, rebaudioside A, steviol (steviol glycosides' aglucon) and isosteviol (steviol glycosides' derivative) have the capability to enhance insulin secretion from isolated mouse islets in a dose-and glucose-dependent manner. 8,9,13,23 In the present study, we have found that the metabolite, SVG, possesses the same properties. In static incubation studies, the insulinotropic effect of SVG was present in a broad concentration range, with the maximal effect at 10 −7 mol/L.…”

“…8,9,13,23 These results emphasize that steviol glycosides have a promising potential in the treatment of T2D, since the insulinotropic action of their metabolic end product, SVG, only operates at high glucose level, but disappears at low glucose concentration, which is indicative of improved glycaemic control with a good safety profile. SVG elicits a typical monophasic insulin response at high glucose of 16.7 mmol/L but had no insulinotropic effect at low glucose (3.3 mmol/L).…”

“…The dynamic insulin response found in the perifusion experiment corroborates with our previous studies with stevioside, rebaudioside A, steviol and isosteviol. 8,9,13,23 These results emphasize that steviol glycosides have a promising potential in the treatment of T2D, since the insulinotropic action of their metabolic end product, SVG, only operates at high glucose level, but disappears at low glucose concentration, which is indicative of improved glycaemic control with a good safety profile. 29 The risk of hypoglycaemia in T2D patients treated with antidiabetic medications, such as sulfonylureas, is a major safety issue that can be fatal.…”

“…Pancreatic islets were isolated from adult female NMRI mice (Taconic) weighing 22-25 g by collagenase digestion technique, as described previously. [21][22][23] In brief, the mice were anaesthetized with pentobarbital intraperitoneally (50 mg/kg…”